This invention relates to heating units employing resistive conductors to construct heat elements, and more particularly, to a microwire staple used to attach and hold such an element in place on a thermal and electrical insulation material.
In electrical heating units of the type used in cook tops for ranges, one type of heating element employed is a resistive conductor exposed to air. When an electric current is passed through the conductor, the power dissipated raises the conductor's temperature. Radiant energy is generated which performs the heating function of the element. In some applications, a thin, elongate strip of a metallic ribbon heating material is passed through a machine which corrugates the material. Such a construction is shown, for example, in U.S. Pat. No. 5,393,958. Other patents of interest with respect to ribbon heating elements include U.S. Pat. 5,453,597, 5,369,874 and 4,161,648. When a current is applied to the ribbon heating element, heat generated by the resulting I.sup.2 r losses is radiated at a utensil set upon the unit. During a heating cycle, the heating element expands and contracts. If the element is constrained to move, stresses are created in the material; and, stress fractures are the primary cause of heating element failures.
It is also known that instead of pressing an edge of a ribbon heating element into the insulation material to lay the heating element on top of the insulation material but to not press it into the material. Rather, metal staples are used to secure the heating element in place. There are, however, a number of problems with this approach as well. First, the staples create non-uniform heating spots because of the concentrated mass of material at each staple location. In addition, the staples obstruct heat radiation to the utensil being heated because the staples absorb the heat radiated by the ribbon heating element in the area around the staple. The conventional wire staples are also quite rigid and they damage the heating element member when pressed too tightly against the heater conductor. The large cross section of the conventional staple does not allow them to be placed close together; this is necessary to avoid short circuits as well as insertion damage to the insulating cake. This leaves large segments of the heater conductor unconstrained and may lead to the unwanted movement of that conductor during handling and transportation. The bulk of the conventional wire staple has excess thermal mass that drains the heat energy from the conductor to raise its temperature. This affects the overall performance of the unit. The conventional wire staple is never able to absorb enough heat from the conductor to become radiant. This together with its large size has a masking effect on the glowing conductor of the heating unit, resulting into dark areas at different locations over the heater geometry. This is aesthetically undesirable.